I've had this problem too. What a trainer recommended to me was to do the exercise starting with the weaker side (in your case, your left). do as many reps as you can while maintaining good form. then you switch to your other arm, and only do as many reps as you did with the other arm.
it will take a while, but eventually things will even out.
i've used this technique with legs too (lunges, single leg squats, etc)